In recent years, hydrogen, which is used as a fuel for fuel cells, receives attention as a clean energy. However, hydrogen causes hydrogen embrittlement in metallic materials such as iron and aluminum alloys. Therefore, it is difficult to efficiently store highly pressurized hydrogen in a storage container for high-pressure hydrogen gas (e.g., a gas cylinder) to be mounted in an automobile. This problem also occurs not only in dominating iron high-pressure gas containers but also in high-pressure gas containers in which an aluminum alloy liner is used for weight reduction purposes. Thus, high-pressure gas containers are required to have excellent resistance to hydrogen embrittlement from the view point of reliability.
In a storage container for high-pressure hydrogen gas using an aluminum alloy liner, it is a mainstream practice to include a composite material produced by winding a fiber-reinforced resin or a fiber for reinforcement around the outer surface of the aluminum alloy liner (filament winding), rather than including an aluminum alloy alone.
For example, Patent Document 1 discloses a method for producing a storage container for high-pressure hydrogen gas using an aluminum alloy liner made of a precipitation hardened 7000-series aluminum alloy extrusion material having high strength. That is, the 7000-series aluminum alloy extrusion material is subjected to drawing, and the drawn product is subjected to a solution heat treatment and then subjected to impact processing, thereby forming a cylindrical body with a bottom. Subsequently, a gas discharge port is formed on the cylindrical body with a bottom by cold forging, and the cylindrical body with a bottom is then aged. In this manner, a small-sized high-pressure gas container is produced.
In Patent Documents 2 and 3, it is proposed to further improve the yield strength of an aluminum alloy liner and also improve a method for producing the liner. That is, it is proposed that a 7000-series aluminum alloy material is subjected to a solution heat treatment and then subjected to ironing while applying a plastic strain to shape the material into the form of a liner, thereby to eliminate an aging treatment to be performed after the solution heat treatment. In Patent Document 3, it is also proposed to use a 7000-series aluminum alloy or an aluminum alloy having a composition standardized to AA6066 (also referred to as “AA6066-standardized aluminum alloy” or an “AA6066 alloy”, hereinafter) which has excellent stress corrosion cracking resistance (SCC resistance) as a liner material.
This type of a liner material for a storage container for high-pressure hydrogen gas including a 6066 aluminum alloy is also proposed in Patent Document 4. Among 6000-series aluminum alloys, the 6066 alloy is a precipitation-hardened aluminum alloy material having high Mg and Si amounts and relatively high strength, and is therefore a promising liner material for a storage container for high-pressure hydrogen gas.
However, if the 6066 aluminum alloy material is used as a liner material regardless of the types of hot processing methods to be employed or the forms of a rolled sheet, an extrusion material, a forged material and the like, the 6066 aluminum alloy material is required to have resistance to hydrogen embrittlement such that the 6066 aluminum alloy material does not embrittle even when the pressure employed for filling a hydrogen gas into a storage container for high-pressure hydrogen gas to be mounted in an automobile is increased. The 6066 aluminum alloy material is also required to have higher strength for the purpose of reducing the thickness of the liner for reducing the weight of the liner.
In this regard, the above-mentioned 7000-series aluminum alloy has higher strength than that of the 6066 alloy. However, the 7000-series alloy contains major elements such as Zn, Mg and Cu at high contents, and therefore has a problem of the occurrence of stress corrosion cracking (SCC) in which hydrogen embrittlement is also involved. Particularly in a high-strength 7000-series alloy of which the strength is increased by performing a peak aging treatment or the like in place of an over-aging treatment, resistance to hydrogen embrittlement is further decreased. Moreover, when the amounts of the major elements added are increased, the cracking of a billet or a slab is likely to occur during melting or casting. In addition, cracking is likely to occur during hot processing such as rolling, forging or extrusion, and during extrusion, problems also occur such that an extrusion rate becomes extremely low, which leads to a reduction in production performance.
For this reason, it can be said that, among aluminum alloys, the AA6066 alloy material is best suited for a storage container for high-pressure hydrogen gas. However, in order to use the 6066 aluminum alloy material as a liner material, it is continuously perceived as an important issue to be improved in both resistance to hydrogen embrittlement and strength.
For this reason, Patent Document 5 proposes that Mg and Si, which are elements contained in AA6066 alloy composition, are contained in amounts falling within specified ranges that satisfy each formula: Mg≦1.73 Si-0.52%, Mg≦1.5%, Mg≧0.9% and Si≦1.8%, thereby improving the resistance to hydrogen embrittlement of the AA6066 alloy.